1. Field of the Invention
The present invention relates to new inhibitors of dimethylarginine dimethylaminohydrolase (DDAH).
2. Discussion of the Prior Art
The DDAH catalyses the physiological decomposition of Nω-monomethyl-L-arginine (NMMA) and Nω,Nω-dimethyl-L-arginine (ADMA) to form L-citrulline and mono- or dimethylamine. These Nω-methylated L-arginines are formed by methylation of protein-bound arginine radicals by the isoenzymes of the protein-arginine-methyltransferases (PRMTs). The protein decomposition releases these two substances and they circulate in physiological concentrations of 0.4 to 1.0 μM in the blood. In pathological conditions however, the values can increase markedly (up to 10 μM).
Both ADMA and NMMA are potent inhibitors of all three isoenzymes of the nitrogen monoxide synthase (NOS) that catalyse the release of nitrogen monoxide (NO) from L-arginine. NO regulates as a potent vasodilatator the vascular tonus, protects against atherosclerotic events and is involved as neurotransmitter in a multiplicity of functions such as memory performance or the formation of pain. In addition, NO has an important role in the unspecified immune defence.
On account of this variety of functions of NO, a balanced regulation of the NO biosynthesis is vital. A reduced NO biosynthesis can lead to cardiovascular diseases such as hypertension, atherosclerosis or coronary heart disease and erectile dysfunction.
On top of this, a multiplicity of pathological states is associated with an overproduction of NO so that an inhibition of NO biosynthesis can be exploited for treating these diseases. Examples, for such pathophysiological states are the disease patterns of the septic shock, the stroke, neurodegenerative diseases, chronic inflammations, rheumatoid arthritis, migraine, inflammatory pain (nociception), diabetes mellitus and meningitis.
Despite the fact that many attempts have been made over the previous years to exploit a direct inhibition of the NO synthase pharmaceutically for treating these disease patterns, none of the tested compounds has been approved as a medicinal product.
The inhibition of DDAH represents a new approach for the indirect inhibition of the isoenzymes of the NO synthase. The inhibition of DDAH leads to a rise in the blood level of Nω-methylated L-arginines and thus to an indirect inhibition of the activity of NOS. In the meantime, several studies confirmed an influence of the NO biosynthesis as a result of a modulation of the DDAH activity. It was thus found in animal models that an inhibition of DDAH leads to a rise in the ADMA blood level and the vascular tonus (Rossiter, Smith et al. 2005; Leiper, Nandi et al. 2007). On top of this, selective DDAH-1 inhibitors are candidates for treating chronic pain and the septic shock (Leiper, Nandi et al. 2007). Inhibition of DDAH can be assumed as a further indication possibility in the treatment of neurodegenerative diseases such as for Morbus Alzheimer, since in the case of this disease pattern, both a reduced ADMA blood level and an increased DDAH activity could be detected.
Over the previous years, a co-expression of DDAH-1 with nNOS and a co-localisation of DDAH-2 with eNOS and partly with iNOS could be detected. As a result, a selective inhibition of DDAH-1 can possibly be used for the selective reduction of the activity of nNOS. In the meantime, use of selective DDAH-1 inhibitors was successfully tested in vivo for treating chronic pain and the septic shock. Involvement of DDAH-1 in tumour growth and the angiogenesis was also detected. Tumour tissue with increased DDAH-1 expression proliferated much faster than other tumour tissue.
On the other hand, inhibition of DDAH-2 makes little, sense, since a reduced endothelial NO release represents a risk factor for cardiovascular complications.
Previously known inhibitors for the human DDAH-1 are the L-arginine derivatives. The at present most potent representative of this group is Nω-(2-methoxyethyl)-L-arginine (L-257, IC50 22 μM, murine DDAH), a guanidine (Rossiter, Smith et al. 2005). On top of this, the Nδ-(1-iminoalk(en)yl-L-ornithines are a very potent inhibitor class with vinyl-L-NIO as the at present most potent known inhibitor of DDAH-1 (Kotthaus, Schade et al. 2008).
It is known to increase the oral bioavailability of the guanidines and amidines by means of prodrug principles. Here, a very successful principle are the N-hydroxyamidines (amidoximes) and N-hydroxyguanidines [Clement, B. DE4321444 and PCT/DE2007/001216].
The inhibitors known so far for human DDAH-1 exhibit bad pharmacokinetic properties due to their amino acid structure since they exist as zwitterions in the case of a physiological pH. The amino function is protonated and thus positively charged, while the carboxyl function is deprotonated and is thus negatively charged. Charged substances are resorbed from the gastrointestinal tract only to a small extent, since they cannot penetrate the lipophile membranes of the gastrointestinal tract by diffusion. The passive diffusion however represents the preferred absorption mechanism of a medicinal product after oral administration. It is thus very unlikely that effective substance levels in the blood are reached after the oral administration of charged compounds. In addition, there is also the possibility of the intake via specific transport systems like the amino acid transporter, however the extent of resorption by means of this mechanism has to be assessed to be much reduced. For this reason, removing charges from a molecule is of eminent importance for the development of medicinal products that are to exhibit an oral bioavailability.